A method for operating a steering system is described, for example, in German Published Patent Application No. 197 51 125. In this instance, the steering motions applied by the driver through a steering wheel, the steering wheel angle, are superposed with the aid of a superposition gear with the motions of an actuating drive or superposition angle actuator to the motor angle or control angle. The superposed motion thus obtained is transmitted via the steering gear or steering linkage to the steerably designed wheels for setting the steering angle. For this purpose, the actuating drive takes the form of an electric motor. The functional principle or the practical applications of such a servo-steering system are, e.g., that the steering can be implemented indirectly through the transformation of the superposition gear and thus low steering wheel torques can be achieved. Very large steering wheel angles, which would result from this, are avoided by superposing suitable control angles such that steering wheel angles of the usual magnitude can be used to set required output angles. The control angle required for steering assistance or its setpoint value is determined from the steering wheel angle. Furthermore, the control angle may also be a function of signals which represent vehicle movements detected by sensors and/or other vehicle systems such as, for example, an electronic stability program (ESP). This is done by a control unit in which the programs required for determining the required motor control angles or for controlling the practical applications are executed.
The control of the control angle and of the output torque of the superposition angle actuator or electric motor of such a superimposed steering system of a motor vehicle normally occurs with the aid of a follow-up control of the position of the control angle and a current control of the output torque as cascade controller.
The following properties of the superimposed steering system are to be considered in the control:                dynamics, i.e., dynamic start-up behavior and response to setpoint changes of the steering;        haptics, i.e., the roughness or waviness of the steering torque (the so-called “ripple”);        acoustics; and        robustness of the superposition angle actuator, i.e., sensitivity with respect to changes of mechanical properties such as static friction or viscous damping as well as environmental influences such as temperature, production inaccuracies and tolerances.        